1. Field of the Invention
The present invention relates to a reformer for producing a synthesis gas from a raw material, such as a natural gas. More particularly, it relates to a heat exchanger type reformer which produces a synthesis gas rich in hydrogen and carbon monoxide by reforming, with steam, a raw material, such as a hydrocarbon gas, by utilizing a high-temperature gas as a heating source.
2. Background of the Invention
There is known a heat exchanger type reformer that utilizes combustion gas or a high-temperature gas obtained from a secondary reformer as a heating source to cause a hydrocarbon to react with steam (reformed) in the presence of a catalyst to produce a gas that is rich in hydrogen and carbon monoxide.
Conventionally, heat exchanger type reformers, as disclosed in U.S. Pat. No. 4,690,690 and Japanese Patent No. 3442167, comprise a bayonet tube, in which an inner tube is inserted in an outer tube wherein the gap between the outer tube and the inner tube is packed with a catalyst. The high-temperature gas is introduced into the reformer from the closed side of the outer tube and caused to flow on the outside surface of the outer tube, by which heat necessary for the reforming reaction is transferred to the raw material gas flowing in a catalyst. In such reformers, one end of the bayonet tube (usually a lower end) is not mechanically fixed, but is free to move in the axial direction of the tube by thermal expansion, which offers an advantage that mechanical problems, such as buckling and tensile fracture caused by thermal expansion of the tube, are less likely to occur. However, loading and unloading of the catalyst is difficult because the catalyst must be loaded into or unloaded from an annular portion between the outer and inner tubes while avoiding an obstacle created by a manifold of the inner tubes in an upper region. Also, the catalyst is sometimes broken during operation because of axial movement caused by a difference in thermal expansion between the outer and inner tubes. Further, since the bayonet tube has a double tube structure, the outline of the tube is thick and the diameter of the reformer is large. And since a raw material gas introduction section and a reformed gas discharge section are provided on one side of the reformer, the reformer is complicated in structure, making it difficult to increase the size of the reformer.
Japanese Patent Laid-Open No. 56-109286, Japanese Patent Laid-Open No. 61-58801, Japanese Patent No. 3450876, and Joshi et al. (“Application of the Kellogg Reforming Exchanger System to Large Scale Methanol Plants”, Girish Joshi et al., 1995 World Methanol Conference Phoenix, Ariz. USA, Dec. 5-7, 1995) have disclosed a reformer in which the above-noted problems are solved. This reformer has a structure in which the reaction tube loaded with the catalyst has a single-tube structure instead of the bayonet tube; one end of the tube opens inside the reformer; the reformed gas discharged from the open end of the tube is mixed with the high-temperature gas supplied from the outside of the system, e.g., from a secondary reformer; and the mixed gas is caused to flow on the outside of the tube, by which heat necessary for the reforming reaction is transferred to the raw material gas flowing in the tube. For this type of reformer, as in the case of the bayonet tube, one end (usually a lower end) of tube is not mechanically fixed, but is free to move in the axial direction of the tube by thermal expansion, which offers an advantage that mechanical problems, such as buckling and tensile fracture caused by thermal expansion of the tube, are likely less to occur. Also, the catalyst is easily loaded and unloaded, and not broken as often during operation. Also, this reformer has a relatively simple structure, thus making it easier to meet the requirement for an increase in size of the reformer. However, the raw material gas supplied into the catalyst layer in the tube undergoes reforming reaction to a certain level and thereafter is usually mixed in the reformer with the high-temperature gas, which has been separately reformed outside the system to a point where residual methane is scarcely present. In addition, after giving off heat necessary for the reforming reaction to the raw material gas in the reaction tube, the mixed gas is discharged from the reformer and sent to succeeding cooling and condensed-water separating processes. Therefore, in this case, the reformed gas is sent to the next process in a state in which a considerable amount of unreacted methane component remains, so that the methane conversion efficiency decreases, while in the case of the reformer using the bayonet tube, the reformed gas coming out of the reformer is further sent to the secondary reformer, and is reformed to a state in which residual methane is scarcely present. Further, the high-temperature gas introduced from the outside of the system gives off heat necessary for the reforming reaction to the raw material gas flowing in the reaction tube after being mixed with the reformed gas having a temperature 150 to 200° C. lower than that of the high-temperature gas thereby lowering the temperature of the high-temperature gas. Therefore, the temperature of the high-temperature gas is hardly utilized effectively, and the heat transfer efficiency decreases. Also, in principle, the reformers of this type cannot be used with a high-temperature gas that cannot be mixed with the process gas (i.e., the raw material gas and the reformed gas obtained by reforming the raw material gas). For example, this type of reformers cannot be used when the high-temperature gas is helium heated by fission heat of a nuclear reactor.
WO 94/29013 discloses a reformer in which the high-temperature gas for giving heat necessary for the reforming reaction is not introduced from the secondary reformer, but from a combustion reaction zone provided in the reformer. In this reformer, the entire construction is a one-pass floating head multi-tubular heat exchanger type, where the catalyst is loaded in tubes and thermal expansion of the tubes is absorbed by two kinds of bellows provided at a floating head portion and at each tube of a fixed-side tube plate portion. For this reformer, however, the floating head has an ordinary floating head structure consisting of a flat-metal tube plate and a metallic floating head cover, and the construction is such that a floating head tube plate is protected from the high-temperature gas introduced into the reformer only by a heat insulating material provided on the back surface of the tube plate. Therefore, high thermal stress is likely to occur at the floating head, so that it is difficult to use this reformer in applications in which the high-temperature gas is introduced directly from the outside of the reformer. Also, the reformer of this type is unsuitable for a large-size reformer because the thermal stress occurring at the floating head is especially remarkable in a large-size reformer.
Reforming catalysts have been disclosed in U.S. Pat. Nos. 4,990,481, 5,100,857, for example.